The present invention relates to a lens provided with an anti-reflecting membrane, and a projection type display apparatus using the same.
Anti-reflecting membranes are formed on the light receiving face and the light outgoing face of lenses used for cameras or telescopes because loss of light occurs due to reflection caused by difference in refractive index at the interface with air, and reflection of ambient objects on the lens surface is large in an environment of strong outer light.
Liquid crystal projectors and rear projection type liquid crystal projection televisions begin to spread in companies and general households as projection type display apparatuses which use as a light source a super high pressure mercury lamp performing irradiation of a large volume of light and which display projected images on a screen through a display device such as a liquid crystal display device.
Briefly explaining the optical system of a liquid crystal projector, the light emitted from a lamp as a light source passes through a plurality of lenses, a polarization converter, a dichroic mirror, a display device and the like, and is synthesized by a dichroic cross-prism, and displayed on a screen through a projection lens.
In the case of the rear projection type liquid crystal projection television, the image light outputted from the projection lens is corrected in its direction by a rear mirror and projected on a screen to display an image.
As mentioned above, a light travels through various optical parts, and hence if reflection at the light receiving face and light outgoing face is great, spectral volume becomes smaller, and, as a result, the image becomes darker. Therefore, anti-reflecting membranes are provided on the light receiving face and the light emitting face of the respective optical parts.
However, most of the anti-reflecting membranes used for the present optical parts are formed by vapor deposition, and hence a vacuum process is needed. Furthermore, the anti-reflecting membranes include multi-layer type and single-layer type, and the single-layer type is preferred because the multi-layer type is superior in anti-reflecting performance, but requires many production steps.
Accordingly, guidelines for designing the anti-reflecting membranes of single-layer type will be explained below.
Materials used for optical parts such as lens include transparent materials such as glass (refractive index: 1.5-1.54), acrylic resins (refractive index: 1.49), PET resins (polyethylene terephthalate resins) (refractive index: 1.56), etc. When refractive index of these materials is shown by n1 and that of air is shown by n0, reflectance R is represented by the following formula.R={(n1−n0)/(n1+n0)}2  (1)
Since the refractive index no of air is usually 1.0, the above formula can be shown by the following formula.R={(n1−1)/(n1+1)}2  (2)
When the refractive index of glass, acrylic resin and PET resin is introduced into this formula, the reflectance at one side of the transparent parts is 3.9-4.0% for glass, 3.9% for acrylic resin and 4.8% for PET resin. When a single-layer anti-reflecting membrane in a proper thickness is provided on the surface of the parts in order to reduce the reflectance, the reflectance R′ can be obtained from the following formula:R′={(n22−n0×n1)/(n22+n0×n1)}2  (3)in which n0 is a refractive index of air layer, n1 is a refractive index of the outermost surface plate, and n2 is a refractive index of the membrane. Since the refractive index no of air is usually 1.0, the above formula can be shown by the following formula:R′={(n22−n1)/(n22+n1)}2  (4)In the case of n22=n1, namely, n2=√n1, the reflectance is theoretically 0%.
When this is introduced into the above formula, the refractive index of the anti-reflecting membrane to be applied to glass is suitably about 1.22. However, as for the present materials, even fluorinated resins which are known to be relatively low in refractive index have a refractive index of about 1.34 and even magnesium fluoride which is known to be particularly low in refractive index among inorganic materials has a refractive index of about 1.38, and thus it is very difficult to obtain sufficient anti-reflecting performance with single-layer membranes.
Recently, methods for further lowering the refractive index of single-layer membranes has been proposed, one of which is use of the aerogel thin membrane disclosed in JP-A-2003-201443. The aerogel thin membrane is a thin membrane comprising fine particles having pores inside (fine hollow particles) and a binder holding the fine hollow particles. The pores in the aerogel thin membrane have a refractive index substantially the same as that of air (refractive index: 1.0), and consequently the membrane has a refractive index near that of air even when the refractive index of the material of the fine hollow particles or the binder holding the fine hollow particles is high. Thus, the reflectance can be reduced by forming the membrane on a plate.
Another method for lowering the refractive index of single-layer membrane, being different from the method of using the aerogel, is to use a membrane of low refractive index disclosed in JP-A-7-92305. It is disclosed that the surface of superfine organic particles is exposed on the side near to air, and irregularities are formed on the surface, thereby reducing the surface density, and, as a result, a membrane of low refractive index is formed.
Furthermore, another method comprises use of membrane of low refractive index having pores in the form of a honeycomb as disclosed in JP-A-2004-83307. According to this method, a plurality of pores in the form of a honeycomb structure are formed so that they pass through fine silica particles and are parallel to each other, whereby maximum pore content can be obtained without reducing the strength of fine silica particles per se. It is disclosed that according to this method, low refractive index membranes excellent in mechanical strength can be formed.
The present multi-layer anti-reflecting membranes prepared using magnesium fluoride, etc. are low in adhesion to a lens having a convex surface or concave surface, and sometimes peel off during long term use. This tendency is conspicuous especially when the material of the membranes is an acrylic resin.
Furthermore, the aerogel thin membrane disclosed in JP-A-2003-201443 has the problem of reduction in mechanical strength due to high pore content. The physical strength of membranes depends greatly upon the physical strength of the fine hollow particles. In order to lower the refractive index, it is necessary to increase the size of pores of the fine hollow particles. Aerogel thin membranes in which the thickness of shell of the fine hollow particles is thin are difficult to increase the physical strength. In case the pore is made larger by increasing the thickness of shell of the fine hollow particles, the size of particles increases and the membrane tends to scatter visible light, resulting in reduction of transmittance, which is practically unacceptable. Further problem is in the specialty of production process which uses carbon oxide of supercritical state. Particularly, for optical parts such as a lens which is not flat, but has irregularities, it is essential to consider the technology of membrane production.
Furthermore, the low refractive index membrane disclosed in JP-A-7-92305 and the low refractive index membrane having pores in the form of honeycomb structure disclosed in JP-A-2004-83307 are considered to have high mechanical strength due to crosslinking or polymerization, but there is the possibility of increase of refractive index owing to incorporation of dusts in the irregularities of the membrane surface. Moreover, in the case of these structures, the refractive index is about 1.3-1.4, which is disparate from the ideal value (lower than 1.3).
The object of the present invention is to solve the above problems. That is, the object is to provide an optical part having an anti-reflecting membrane which has both the high anti-reflecting performance and the high physical strength, and a display apparatus using the same.
Furthermore, when they are disposed under conditions of high temperature and high humidity such as hot-spring hotels and oceanfront hotels, the optical parts such as lenses and prisms in the display apparatuses get mildewed to cause decrease of light transmission. Therefore, the present invention discloses also an optical part having anti-reflecting membrane further imparted with mildew proofing function, and a display apparatus using the same.
As a result of intensive research conducted by the inventors on various materials for formation of membranes and method for formation, a method has been found according to which pores having a size of 5-200 nm are provided in a binder or between the binder and inorganic oxide particles in a membrane formed of inorganic oxide particles and a binder. It has also been found that the above membrane has a refractive index smaller that that of the binder and is excellent in physical strength because of using inorganic oxide particles having no pores although the membrane has pores inside, and further is high in adhesion to the transparent materials such as acrylic resins and glasses, and it has been found that optical parts such as lens which have the above membrane are less in reflection at the surface and high in light transmission, and besides high in physical strength since they are superior in adhesion to the surface of the membrane.
Furthermore, it has been found that the pores in the membrane are not present uniformly, but are localized in the vicinity of the surface of the anti-reflecting membrane rather than in the vicinity of the surface of the plate, and hence even if there are some differences in thickness of the membrane, the anti-reflecting function is exhibited and this function is obtained for light of a wide wavelength region.
Moreover, it has also been found that the membrane in which fine silicon dioxide particles are used as the fine inorganic oxide particles and a silicon compound having a hydrolysable residue (silica sol) is used as the binder shows a refractive index considerably smaller than that of silicon dioxide as the binder (the refractive index is specifically 1.33 or smaller) and is excellent in physical strength although it has pores inside.
In addition this membrane is also high in adhesion to the plate. Particularly, when the material is a resin such as an acrylic resin, the membrane is conspicuously higher in the adhesion as compared with the conventional anti-reflecting membranes comprising magnesium fluoride, fluorinated resins, or the like.
Moreover, since this membrane has a very small surface resistance, there is exhibited an effect that dusts such as dirt hardly adhere to the membrane even under low temperature conditions such as winter.
When they are disposed under conditions of high temperature and high humidity such as hot-spring hotels and oceanfront hotels, the optical parts such as lenses and prisms in the display apparatuses sometimes get mildewed to cause decrease of light transmission. Therefore, the present invention has an additional object to impart mildew proofing function to the anti-reflecting membrane formed on the optical parts such as lenses and prisms.